Discovery completes missing link in the evolutionary transition of the alvarezsaurians – ScienceDaily

An international team of researchers discovered a new kind of dinosaur, Xiyunykus pengi, during an expedition to Xinjiang, China. The discovery is the final result of a collaboration between George Washington University and the Chinese Academy of Sciences. The findings were published today in Current biology together with the description of a second new intermediate type, Bannykus wulatensis.

Xiyunykus and Bannykus both are alvarezsaurs, a mysterious group of dinosaurs that share many traits with birds. Their bodies are slender, with a bird-like skull and many small teeth instead of the usual large, sharp teeth of their carnivorous family members.

"When we described the first known alvarezsaur, Mononykus, in 1993, we were amazed at the contrast between his mole-like arms and his roadrunner-like body, but there were few fossils linking it to other theropod groups," James Clark, the Ronald Weintraub professor of biology at the GW Columbia College of Arts and Sciences, said.

However, Alvarezsaurs did not always look like this. Early members of the group had relatively long arms with strongly clawed hands and typical carnivorous teeth. Over time, the alvarezsaurians evolved into dinosaurs with mula-like arms and a single claw. The discovery of the new specimens enabled the researchers to expose an important shift in how the specialized characteristics of the alvarezsaurians evolved.

"It can be difficult to identify the relationships of highly specialized animals, but fossil species with transitional characteristics such as Xiyunykus and Bannykus, are extremely useful because they link bizarre anatomical features to more typical ones, "says Jonah Choiniere, associate professor at the University of Wits and a member of the research team.

The fossils were discovered during an expedition led by Dr. Clark and Xing Xu from the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences. Xiyunykus pengi is the ninth kind of dinosaur identified by the partnership between GW and the academy.

"Our international field teams have been hugely productive over the years," Dr. said. Xu. "This research shows just a few of our incredible discoveries."

The fieldwork and research were supported by the US National Science Foundation Grant EAR 0922187.

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